Formatted Title
Colloidal Carbon: Protecting Drinking Water Wells from a UST Release in a Weathered Aquifer
Background/Objectives
An active gas station in Muscle Shoals, Alabama, was impacted by a UST release threatening eight nearby private drinking wells. The discovery of the release occurred when a nearby resident detected a strong petroleum odor in their private well water, located about 450 feet downgradient from the release point. Dissolved phase benzene concentrations exceeded the Tier II site-specific limits at eight monitoring well locations on and off-site. The geology governing the plume was complex, resulting in an off-site, diving plume. The objective of the remedial effort was to reduce petroleum hydrocarbons (PHCs), particularly benzene, to below Tier II site-specific levels (10 ppb for benzene) at each of the site monitoring wells while also allowing for the continued use of eight local drinking wells. State regulators initially evaluated a more familiar technology, ozone, as the remedy. However, upon further assessment, ozone treatment was estimated to be prolonged and lacking sufficient distribution, which made activated carbon treatment and the potential for immediate results an attractive remedial option.
Approach/Activities
Colloidal activated carbon (CAC) was selected as a remedy to address the plume source and also prevent plume expansion to other residential wells. The small particle size of CAC allowed for the low-pressure flooding of the pre-existent flux zones naturally found in the weathered aquifer (i.e., sand, clay, and limestone) and was combined with nitrate and sulfate to stimulate anaerobic bioremediation of captured PHCs.
Prior high-resolution site characterization (HRSC) efforts provided the basis for the first site conceptual model, further refined with passive flux tracers and hydraulic radius of influence (ROI) testing. The ROI testing was performed by direct push technology (DPT) injection of clean water to help confirm the ability to place CAC in the key flux zones. These efforts significantly enhanced reagent emplacement within the aquifer.
In August 2021, the full-scale application was completed, consisting of 82,400 lb of CAC with a DPT application method. This material was strategically applied at the site using a tight grid formation in the former underground storage tanks (i.e., the source zone) plus nine downgradient barriers to halt further plume migration.
Results/Lessons Learned
Post-application monitoring demonstrates significant reductions in benzene and PHC concentrations. Seven of the eight monitoring wells achieved the treatment goals within three months following the application. These reductions have been maintained in the two years following treatment with an average contaminant reduction greater than 97%. One monitoring well, located in the most upgradient portion of the off-site treatment zone, has achieved an 89% reduction in total PHC mass with benzene concentrations slightly exceeding the Tier II standards (i.e., 16 ppb). However, monitoring wells located downgradient of the nine-barrier system have gradually decreased to non-detect.
The results from these efforts highlight the importance of HRSC to enhance the site conceptual model and the utility of pre-application hydraulic testing to optimize remediation for complex sites such as this. The post-application data demonstrated that the CAC amendment prevented the continued spread of the benzene plume. This strategic remedial program underscores the need for a combined source zone and downgradient treatment system to reduce and eliminate plume strength and mobility. The unique characteristics of CAC make it a well-suited technology in complex geological environments to provide immediate and long-term treatment at similar sites. Remedial injectant and implementation impacts on the final benzene reductions seen after two years will be presented. Further work on-site to achieve closure at the noncompliant well will be further explored.